It is known in the art of medical radiography to employ intensifying screens to reduce the X-ray dosage to the patient. Intensifying screens absorb the X-ray radiations and emit electromagnetic radiations which can be better absorbed by silver halide emulsion layers. Another approach to reduce the X-ray dosage to the patient is to coat two silver halide emulsion layers on the opposite sides of a support to form a duplitized radiographic element.
Accordingly, it is a common practice in medical radiography to use a radiographic assembly consisting of a duplitized radiographic element interposed between a pair of front and back screens.
The typical structure of an intensifying screen comprises a support and a phosphor layer coated thereon. The phosphor layer comprises a fluorescent substance able to emit light when exposed to X-ray and a binder. Additionally, a primer layer is sometimes provided between the fluorescent layer and the substrate to assist in bonding the fluorescent layer to the substrate, and a reflective layer is sometimes provided between the substrate (or the primer) and the fluorescent layer. Finally, a protective layer for physically and chemically protecting the screen is usually provided on the surface of the fluorescent layer.
Typically, polymer materials, such as polyethylene terephthalate, or paper are used as support for the intensifying screen. Intensifying screens obtained from such supports easily can be electrostatically charged on its surface due to repeated physical contacts with other surfaces of different materials during their use. This static electrification can promote some adverse effects in practical operations of radiation image recording and reproducing.
For example, when the surface of an intensifying screen is charged, it may adhere to another screen or to a radiographic film coupled with it during the exposure of the patient to X-rays. The resulting image provided by the film can suffer of static marks when discharge of the panel takes place. The static marks are produced in the form of over-exposed portions on the radiographic film in contact with the intensifying screen, corresponding to areas in which discharge of the static electricity takes place. Static marks appearing on radiographic films are disadvantageous, in particular in medical radiography for diagnosis, where static marks cause problems in the analysis of the resulting photographic image.
A number of patents and patent applications have been issued on this problem, offering a number of solutions.
JP 03/255,400 discloses an intensifying screen comprising a protective layer of fine particles of metal oxides dispersed in a binder.
JP 03/252,599 discloses an intensifying screen comprising a protective layer consisting of an N-heterocycle compound dispersed in cellulose acetate.
JP 03/237,399 discloses an intensifying screen comprising an intermediate conducting layer between the support and the fluorescent layer consisting of carbon black and/or metals dispersed in a binder.
EP 223,062 discloses an intensifying screen comprising a intermediate or back layer comprising metal oxides, carbon black, or conductive organic compounds.
U.S. Pat. No. 5,151,604 discloses an intensifying screen comprising a subbing layer interposed between the support and a fluorescent layer comprising conductive ZnO whiskers having average diameters of 0.3 to 3.0 mm and average lengths of 3 to 150 mm.
U.S. Pat. No. 4,943,727 discloses an intensifying screen comprising a protective layer having on one or both surfaces thereof a metallic film obtained by evaporating a metal compound selected among Ni, Cr, Au, Sn, Al, Cu, and Zn.
U.S. Pat. No. 4,711,827 discloses an intensifying screen comprising an acrylo-nitrile/styrene copolymer composition as protective top-coat.
U.S. Pat. No. 4,666,774 discloses an intensifying screen with a protective layer of a fluorinated polymer comprising an antistatic agent selected from the group of alkylphosphate mixtures, quaternized fatty imidazine derivatives, and ethoxylated amines.
U.S. Pat. No. 4,983,848 discloses an intensifying screen having a top-coat layer consisting of polyamide derivatives, such as, nylon 6,6, nylon 6, amorphous nylon and the like.
U.S. Pat. No. 4,855,191 discloses an intensifying screen with an antistatic layer comprising a conductive polymer layer, such as acrylic resins or polysiloxanes.
EP 377,470 discloses an intensifying screen comprising an antistatic topcoat layer having inorganic salts dispersed in a binder. Preferred inorganic salts are, for example, LiCl, NaCl, NaBr, NaNO.sub.3, Na.sub.3 PO.sub.4, Csl, MgBr.sub.2, BaBr.sub.2, BaI.sub.2, AlBr.sub.3.
In spite of this activity to solve the long-standing problem of static marks, a definitive solution is still to be reached. It is an object of the present invention to contribute to the reduction of static marks on photographic films, particularly those intended to be used in medical radiography.